writeback: add missing requeue_io in writeback_inodes_wb

In "writeback: fix writeback_inodes_wb from writeback_inodes_sb" I
accidentally removed the requeue_io if we need to skip a superblock
because we can't pin it.  Add it back, otherwise we're getting spurious
lockups after multiple xfstests runs.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jens Axboe <jaxboe@fusionio.com>